The invention relates to magnetic resonance imaging, and more particularly relates to pulse sequences used in magnetic resonance imaging. In its most immediate sense, the invention relates to pulse sequences for acquisition of proton density weighted images and T.sub.2 -weighted images.
In spin echo and gradient spin echo sequences, the amplitude of the echo signal(s) which follow each refocussing pulse progressively decreases with time as a result of spin-spin (T.sub.2) relaxation. As a result, the signal-to-noise ratio of the (gradient) spin echo signals progressively deteriorates.
This phenomenon has disadvantageous consequences when, as is common in clinical practice, a single pulse sequence is used to acquire data to reconstruct a proton density weighted image and a T.sub.2 -weighted image. The proton density weighted image data is acquired at the beginning of the sequence, commencing after the first 180.degree. RF refocussing pulse. During this acquisition, the amplitude of the echo signal is at a maximum and the signal-to-noise ratio is therefore high. The T.sub.2 -weighted image data, on the other hand, are acquired later on, when the amplitude of the echo signal is lower. Therefore, the signal-to-noise ratio of the T.sub.2 -weighted image data is poorer than that of the proton density weighted image data. As a result, the T.sub.2 -weighted image is noisier than the proton density weighted image.
It would be advantageous to improve the quality of a T.sub.2 -weighted image which is produced during the same pulse sequence as a proton weighted image.
In accordance with the invention, the gradient read time is varied during the course of a single pulse sequence in which both proton weighted image data and T.sub.2 -weighted image data are collected. In accordance with the preferred embodiment, the gradient read time used to collect the T.sub.2 -weighted image data is longer than the gradient read time used to collect the proton density weighted image data. This may be accomplished by increasing the gradient read time at each phase-encoding gradient and by acquiring a larger number of T.sub.2 -weighted signals and averaging them.
By using a longer gradient read time during collection of T.sub.2 -weighted image data, the bandwidth of this data is narrowed and the signal-to-noise ratio of the data is correspondingly improved. This compensates for the intrinsically poorer signal-to-noise ratio of the echo signal and makes the signal-to-noise ratio of the T2-weighted image comparable to the signal-to-noise ratio of the proton density weighted image.
In accordance with another aspect of the invention, the number of gradient refocussings after each RF refocussing pulse in a gradient spin echo pulse sequence is changed during the course of the pulse sequence. This makes it possible to acquire the necessary proton density weighted image data and the T2-weighted image data more rapidly than in conventional spin echo imaging.
In preferred embodiments, the gradient read time can be varied not only by changing the duration of gradient read-out at a particular phase-encoding gradient but also by changing the number of echo signals which are read out after the appropriate RF refocussing pulses, i.e. by changing the number of gradient refocussings. It is also possible to use a mixed-type pulse sequence in which e.g. the proton density weighted image data is acquired using multiple gradient refocussings while the T.sub.2 -weighted image data is acquired at a read-out gradient which remains constant between one RF refocussing pulse and the next one.